The present invention relates to a fluid flow control valve, especially for use in heating installations of motor vehicles, which is arranged in a conduit feeding water to a heat exchanger and in which the means for moving the valve member between an open and a closed position comprise an electromagnet controlled by a temperature regulating device. Such a value is for instance disclosed in the U.S. Pat. No. 4,058,255, in which a working space of the valve can be placed either at atmospheric pressure or at a pressure below the atmospheric pressure so as to move the valve member at a pressure below atmospheric pressure in the working space against the action of a compression spring to the open position. The pressure in the working space is controlled by an electromagnet cooperating with a closure member for closing and opening a conduit communicating with the working space and adapted to place the interior thereof at a pressure below atmospheric pressure. In heating arrangements for motor vehicles the flow of water to the heat exchanger is controlled in dependence on the temperature in the interior of the vehicle and the temperature of the heat exchanger. This requires a fluid-flow control valve which carries out regulating impulses imparted thereto from a temperature regulator with the smallest possible delay. Especially in divided heat-water circuits, that is a separated heat-water circuit for the left and the right half of the vehicle, the amount of water for heating passing through each of the circuits is only half of the total amount of water. In such an arrangement an exact sensitivity of response of the valve is necessary for proper dosage of each circuit. It is also desirable that during the operation of the vehicle, the continuously changing conditions, such as water pressure and system pressure, have the least possible influence on the fluid-flow control valve. It is further desirable that the danger of leaks and resulting water loss is definitely prevented. In addition it is desirable that the fluid-flow control valve will properly function even if the water passing therethrough carries impurities, such as dirt or sand. It is further desirable that the fluid-flow control valve is maintained in its open position when the electromagnet controlling the valve member is deenergized and that the energy requirements for energizing the electromagnet be as small as possible and that the valve may be constructed in a compact manner of relatively small weight and with perfect safety of function. All these criteria are not accomplished by the fluid-flow control valve disclosed in the aforementioned patent.